1. Field of the Invention
The present invention is related to a power converter control method and more particularly applied in a parallel-connected operation, thereby automatically balancing the load current distribution among power converters and reducing the total harmonic distortion of output voltage under the nonlinear load.
2. Description of the Related Art
In order to provide high capacity and high quality of an AC electric power, it can be implemented by using a high-capacity power converter or several low-capacity power converters. In the views of the extensible ability and the reliability of a power system, the parallel-connected operation of several low-capacity power converters is better than the singular operation of one high-capacity power converter.
The major problem for parallel-connected operation of several low-capacity power converters is the unbalanced current distribution among power converters. The unbalanced current distribution of power converter may result in over-current of the individual power converter. Besides, the parallel-connected power converters may result in the circulating current among them. It may result in increasing the power loss in the power converters, reduction of the power efficiency of the entire parallel-connected system, and over-current of the individual power converter.
The conventional control methods for the operation of parallel-connected power converters include the master-slave control method, the concentrated distribution control method, the logical distribution control method, a ring control method and a drooping control method.
In the master-slave control method, one of the parallel-connected power converters is regarded as a master power converter and it is controlled by the voltage mode control, thereby supplying a regulated output voltage with low total harmonic distortion. Meanwhile, the other power converters are operated as slave power converters controlled by the current mode control to share the equalized distribution current. A load current distribution circuit is used to decide the output current of each power converter. Thus, it requires the communication signals of current distribution information among the power converters.
The concentrated distribution control method is similar to the master-slave control method. The difference between the concentrated distribution control method and the master-slave control method is that all of the parallel-connected power converters of concentrated distribution control method are operated in the current mode control. A current distribution circuit is used to decide the output current of every power converters. Each of the parallel-connected power converters is controlled by an outer voltage control loop and an inner current control loop, thereby supplying a regulated output voltage with low total harmonic distortion. In this method, the parallel-connected power converters system will be failed if the current distribution circuit is failed.
The logical distribution control method uses a control circuit to detect the output current of each power converter. This control circuit collects the output currents of all power converters and averages them to determine the reference output current of each power converter. Then the control circuit, using the current control mode, controls the output current of each power converter to be identical to the reference output current. Hence, the output current of each power converter is regulated to be equal. In this method, the output currents of all parallel-connected power converters must be collected and the numbers of power converter to be paralleled must be pre-known. If any of the parallel-connected power converters is failed, the parallel-connected system will be failed.
The ring control method is similar to the logical distribution control method. However, a control circuit merely retrieves the output of one preceding member of power converter other than all of them and regards it as a reference signal in the ring control method. As the first member of the power converter, it retrieves the output of the last member and regards it as a reference signal. All of the power converters constitute a ring structure. The power converters are controlled by the current mode control to guarantee its output current is identical to the reference signal and supplied a regulated output voltage with low total harmonic distortion. Thus, it requires to get the output current signal of the preceding power converter and to supply the output current signal to the next power converter.
In the above control methods, the current distribution information must be communicated among the power converters. The communication information is easy to be interfered. Therefore, the reliability of parallel-connected power converters will be degraded.
Another control method for parallel-connected power converters is the drooping control method. The conventional P-Q control method is used in this method. The P-Q control method controls the real power supplied from a power converter by means of controlling its voltage phase and reactive power supplied by means of controlling its voltage amplitude. To obtain a stable parallel-connected power converter system, the drooping control method controls the phase (frequency) and the amplitude of the power converter in a negative-slope relationship. In the drooping control method, the output voltage of power converter may be decreased while the output current is increased, and thus the current supplied from each power converter is varied. Equal current distribution supplied from each power converter becomes well if the drooping slope is increased and thus its voltage regulation is degraded. On the contrary, the voltage regulation is improved if the drooping slope is decreased and thus the equal current distribution becomes poor. Although the drooping control method does not use communication signals of distribution current information among the power converters, it has drawbacks that the output current of each power converter is not the same and the voltage regulation is degraded.
The present invention intends to provide a control method for parallel-connected power converters. This method does not use the communication of distribution current signals among the parallel-connected power converters to increase the reliability. Moreover, it can effectively improve the voltage regulation at load terminal, unify the equal current distribution of the output current among power converters and reduce the total harmonic distortion of output voltage under a nonlinear load.